Plasma based light sources are commonly implemented to generate extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) light in wafer and mask inspection systems. For instance, discharged-produced plasma (DPP) and laser-produced plasma (LPP) are conventional components in EUV illumination source technologies.
DPP and LPP based light sources possess a number of disadvantages. For example, in order to achieve higher levels of in-band EUV brightness, time-multiplexing of several sources is required, particularly in technologies implementing DPP sources. The time-multiplexing of several sources limits inspection throughput, while simultaneously increasing cost and complexity.
In addition, spectral output in DPP and LPP sources cannot be directly controlled in design or operation. In the case of actinic EUV mask inspection, target materials are typically selected based on the existence of transition radiation within the 2% bandwidth defined by achievable multilayer coating designs, and plasma temperatures are optimized for overall conversion efficiency. Further, a low cost LPP source suitable for actinic EUV mask inspection can have different spectral properties from a scanner high-power LPP source, thereby increasing the difficulty of relating inspection image information to defect printability. In the case of broadband wafer inspection systems, such as VUV systems operating with light in the VUV (100-200 nm) region, a broad spectral emission combined with selectable bandpass filtering is typically required. It is noted that few target materials display broad VUV emission, forcing consideration of multiple target materials, often with vastly different physical properties.
In addition, pulse repetition rate in LPP and DPP systems is limited by a number of physical constraints, with pulse-pulse energy fluctuations being significant. Accordingly, system energy/dose monitoring and image normalization must achieve higher levels of performance in LPP and DPP systems, as compared to continuous wave (CW) or quasi-CW sources commonly implemented in deep UV mask inspection systems. Due to the inherent variability of drive energy transport or timing, and target material conditions, pulsed plasma sources have undesirable pulse-pulse variations of emission energy and spatial distribution. Since source repetition rates are typically below 50 KHz, and as low as 2 KHz, while integration times of inspection systems range from 1-10 ms, inspection system design must accommodate large variability in the number of pulses, and usually must incorporate a homogenizer function within the illumination optics to mitigate fluctuations of the in-band emission spatial distribution.
Further, surfaces and materials in close proximity to the plasma of a LPP or DPP source are typically exposed to high energy neutral and ion flux, which sputters and spalls the material. As such, debris and contamination mitigation is typically required to protect the associated downstream optics and mask. Given the ultraclean environment requirement for EUV mask inspection/metrology, this can be both expensive and difficult to implement. The debris is especially problematic for DPP sources as the particle production rate for a typical DPP source is on the order of cubic centimeters per week.
As such, it is desirable to provide an illumination source and corresponding mask and/or wafer characterization system (e.g., actinic EUV mask inspection system, wafer inspection system, EUV mask metrology system and the like) that overcomes the deficiencies identified in the prior art, enabling a clear path to ever-increasing demands on next generation systems.